Electrostatic discharge (ESD) is a continuing problem in the design, manufacture, and utilization of integrated circuits (ICs). A major source of ESD exposure is from the human body (described by the “Human Body Model”, HBM). In this situation, a packaged IC acquires a charge when it is touched by a human who is electrostatically charged (e.g. from walking across a carpet). A charge of about 0.4 uC may be induced on a body capacitance of 100 pF, for example, leading to an electrostatic potential of 4 kV or more and discharge peak currents of several amperes to the IC for longer than 100 ns. A second source of ESD exposure is from charged metallic objects (described by the “Machine Model”, MM), which is characterized by a greater capacitance, lower internal resistance and transients that have significantly higher peak current levels than a HBM ESD source. A third source of ESD exposure is due to the discharge to ground of stored charge on the integrated circuit itself (described by the “Charged Device Model”, CDM), with rise times of less than 500 ps. The current flow during CDM is in the opposite direction than from the HBM and MM ESD sources. For all three sources of ESD exposure, both positive and negative polarity discharges may occur.
A current vs time plot of a typical HBM ESD event is shown in FIG. 1B. The current 100 (or voltage) rises very rapidly. An ESD circuit 204 connected in parallel to an input/output transistor 202 of an integrated circuit 212 (FIG. 1A) and ground 210 detects the ESD event and turns on to rapidly shorts excess charge to ground 210 before it can damage the integrated circuit (IC) 212. A driver circuit 200 included in the IC 212 drives the gate of the input/output (I/O) transistor 202.